This invention is in the field of thermoplastic polymeric films which may be used for the packaging of products, especially food products such as meat and cheese. More specifically, the invention is in the field of polymeric thermoplastic films that are thermoformable i.e., may be softened through the application of heat, distorted to a desired shape, and cooled.
Polymeric films are widely used in the packaging field for the packaging of products, especially food products. Films used for the packaging of food generally contain multiple layers, in which each layer adds certain desired physical or chemical properties to the completed film. For example, an xe2x80x9coxygen barrier layerxe2x80x9d serves to protect the packaged food from spoiling. Oxygen barrier properties are necessary to protect many packaged foods over an extended shelf life. Additionally, an xe2x80x9cabuse layerxe2x80x9d may serve to protect the packaged product from physical abuse or stresses caused by the normal handling of the product during packaging, shipping, or during commercialization.
Although each of the layers of a polymeric film serve a particular purpose, the film itself must be stable, and the layers must remain together and not delaminate.
One polymer typically used as a barrier layer is ethylene vinyl alcohol copolymers (xe2x80x9cEVOHxe2x80x9d) which are also known as saponified or hydrolyzed ethylene vinyl acetate copolymers. In addition to its desirable properties as a barrier to oxygen and other gases, EVOH is also an effective barrier to odors, fragrances, solvents and oils. EVOH also has good properties of processability, i.e. in comparison with other polymers it is relatively easy to process into a layer of a multilayer film. EVOH and EVOH copolymers are generally classified according to ethylene content, for example by mole percent. Typically, as the percentage of ethylene increases in relatively low humidity applications, the gas barrier properties decrease, moisture barrier properties improve, and the resin is more processable. At higher levels of humidity, for example at levels that are common for the packaging of meat, of from 90% to 92% humidity, higher ethylene content results in an increase in the moisture barrier properties.
However, there are in general some difficulties with the use of EVOH copolymer in polymeric films. For example, in comparison with other resins EVOH copolymers have poor impact resistance, poor flex crack resistance, and poor drawability.
In order to protect the EVOH layer, which is as previously noted poor in impact and flex crack resistance, additional layers must be added to cover the EVOH layer. One such layer which may be used to protect the EVOH layer is a layer comprising a polyamide. A polyamide is a high molecular weight polymer having amide linkages along the molecular chain structure. Nylon polyamides, which are synthetic polyamides, have favorable physical properties of high strength, stiffness and abrasion resistance.
It is known in the art of making polymeric films to construct a multilayer film with a barrier layer of EVOH copolymer sandwiched by layers comprising a nylon polyamide. The following patents are of interest in illustrating the prior art polymeric films having a core layer of EVOH between two layers of a nylon,
Commonly owned U.S. Pat. No. 4,284,674, to Sheptak, teaches a polymeric thermal insulation product with EVOH core layer adhered on each side to a nylon layer. The nylon layer is adhered to another layer of a polyolefin, such as an oriented polypropylene.
Commonly owned U.S. Pat. No. 4,355,721, to Knott, teaches at column 5, lines 44 to 52, a multilayer polymeric film for food packaging having a core barrier layer of EVOH sandwiched between two nylon layers. An adhesive layer and HDPE sealant layer are disposed on one side of one of the nylon layers.
Commonly owned U.S. Pat. No. 4,640,852, to Ossian, discloses a multilayer polymeric film with a core layer of EVOH sandwiched between two nylon layers. The film may have additional fourth and fifth layers of an adhesive and a heat sealable polymer, disposed on one side of the nylon layers, and sixth and seventh layers of an adhesive and a LLDPE or propylene-ethylene copolymer disposed on the other nylon layer.
U.S. Pat. No. 4,695,491, to Kondo, teaches a heat shrinkable composite packaging film. The film has a core layer of EVOH which is surrounded on one side by a first layer of a polymer having a low permeability to hot water, and on the opposing side an outermost layer of antiblocking polymer. The outermost layer may be a nylon.
U.S. Pat. No. 4,729,926, to Koteles, discloses at column 4 a polymeric film having the following structure:
tie/nylon/EVOH/nylon/tie/LDPE/primer/outer layer
The outer layer may be any of various polymeric materials that are suitable barrier materials.
U.S. Pat. No. 4,735,855, to Wofford, teaches a seven-layer thermoformable polymeric laminate having the following layer structure:
abuse layer/tie/nylon/EVOH/nylon/tie/sealant
The sealant layer may be any of various sealants, including an ionomer. The abuse layer also serves as a moisture barrier layer.
U.S. Pat. No. 4,746,562, to Fant, discloses a seven-layer polymeric film having the following layer structure:
LLDPE/tie/nylon/EVOH/nylon/tie/LLDPE
Each of the outer LLDPE layers also comprises an anti-blocking agent.
U.S. Pat. No. 4,755,419, to Shah, discloses an oxygen barrier oriented seven layer heat-shrinkable film of the following layer structure:
blend/tie/nylon/EVOH/nylon/tie/blend
The blend used for the outer layers may be a blend of LLDPE, LMDPE and EVA. Alternatively, the outer layers may comprise a blend of ethylene propylene copolymer or polypropylene.
U.S. Pat. No. 4,788,105, to Mueller, teaches an oxygen barrier film which is adhered through use of an adhesive layer to a second film comprising a nylon. The film may also comprise an LLDPE outer layer.
U.S. Pat. No. 4,816,304, to Nohara, teaches a multilayer gas barrier vessel with a core layer of EVOH sandwiched between two nylon layers, an outer layer of polyester, and an inner layer of polyester.
Commonly owned U.S. Pat. No. 4,818,592, to Ossian, teaches a core layer of EVOH sandwiched between two nylon layers. The film may also include a fourth layer of an adhesive and a fifth layer of a heat sealable polymer. In an alternative embodiment the film may also include a sixth layer of an adhesive and a seventh layer of a heat sealable polymer disposed on the fifth layer.
U.S. Pat. No. 4,833,696, to Iwanami, discloses a laminate with improved flex crack resistance, drawability and modality, and excellent gas impermeability. The laminate comprises a first layer of an EVOH-copolymer and a thermoplastic polyester, and a second layer of a composition consisting of a group including nylon.
U.S. Pat. No. 4,855,178, to Langley, discloses a chemical barrier fabric in which a fabric material is laminated to a multilayer film of a layer of EVOH sandwiched between two layers of nylon. A heat-sealable polyethylene layer is disposed on the outside of the sheet material.
U.S. Pat. No. 4,909,726, to Bekele, teaches a multilayer polymeric film for chub packaging of the following layer structure:
heat seal/abuse layer/tie/nylon/EVOH/nylon/tie
The heat seal layer may be selected from the group consisting of ethylene alpha-olefin copolymer, LDPE, and ethylene ester copolymer, and may include an antiblocking agent. The abuse layer may be a very low density polyethylene or alternatively may be an ionomer.
U.S. Pat. No. 4,983,431, to Gibbons, teaches in FIG. 3 a five-layer polymeric film which is laminated to a substrate such as paperboard. The film has the following layer structure:
ionomer/nylon/EVOH/nylon/ionomer
One ionomer layer is laminated to the substrate while the second ionomer layer is coated with a layer of LDPE.
U.S. Pat. No. 4,937,112, to Schirmer, teaches a multilayer blown polymeric film for use in chub packaging. The film has a first outer layer of a heat sealable polymeric resin, such as LLDPE, a first interim layer comprising a polymeric material of high molecular weight such as LDPE, HDPE or EVA, a second interim layer comprising a nylon, and a second outer layer comprising a self-weldable polymeric material.
U.S. Pat. No. 4,999,229, to Moritani, teaches a multilayered polymeric gas barrier film with an intermediate layer of a composition having 50 to 97 weight percent EVOH and 45 to 3 weight percent nylon. As disclosed in Example 1, a layer of nylon may be disposed in contact with the intermediate layer.
U.S. Pat. No. 5,068,077, to Negi, teaches a multilayer polymeric film with a barrier layer of from 70 to 95 weight percent EVOH and from 5 to 30 weight percent nylon. The barrier layer may be sandwiched between two layers of nylon.
U.S. Pat. No. 5,194,306, to Blatz, teaches a polymeric blend of a major portion of an amorphous nylon and a minor portion of EVOH for use as a gas barrier in a multilayer polymeric film. In an alternative embodiment at column 6, lines 8 to 11, the invention may comprise a two layer structure with one layer of substantially EVOH and one layer of substantially amorphous nylon. At column 8, example 20 discloses a three layer structure comprising a core layer of EVOH sandwiched between two nylon layers.
It has now been found that a clear multilayer polymeric film of a unique structure, including a core layer of EVOH sandwiched between two nylon layers, or two nylon inner layers and a nylon outer layer, represents an improved multilayer thermoformable polymeric film. The films of the invention are coextruded and then blown into a tubular shape. The films are then cooled by the method of water quenching.
The films of the invention have improved physicals properties of xe2x80x9csnap backxe2x80x9d or xe2x80x9cmemoryxe2x80x9d over prior art films. The improved physical properties mean that after the film is used in the packaging of a product the film shrinks or tightly wraps around the product. The films of the invention create a much tighter package than prior art films, and retain tightness over a longer period of time than prior art films.
The films also have improved properties of gloss, and higher clarity and lesser haze than the prior art. The resulting film of the invention produces a packaging material having an improved structure and appearance. The advantages of the films of the invention are especially apparent in thermoforming applications.
The improved physical properties and appearance of the films of the invention are believed to result from the water quenching method in which the films are made. The films are cooled by the application of air as the coextruded film leaves the die. After air cooling the film is cooled by the application of water in direct contact with the film.
In a first embodiment, the film comprises a first core oxygen barrier layer of an EVOH copolymer. The EVOH may comprise up to 44 mole percent ethylene. The EVOH layer is sandwiched between second and third inner layers comprising a nylon, preferably 5 to 35 weight percent of an amorphous nylon copolymers that is blended with one or more various other nylons. Each of the two inner nylon layers may also include a nucleating agent.
Fourth and fifth layers of an adhesive or tie resin are disposed in contact with each of the second and third layers of nylon, respectively. Suitable adhesive resins include anhydride modified polyolefins, such as EVA-based or LLDPE-based adhesives, or any of various other polymeric adhesives that are commonly used in the art of making polymeric films.
Disposed on the fourth adhesive layer is a sixth outer layer of nylon. Similar to the second and third layers of nylon, the sixth layer of nylon preferably comprises from 5 to 35 weight percent of an amorphous nylon copolymer that is blended with other nylons, and may further include a nucleating agent and an antiblocking agent.
The seventh layer of the film, which is disposed in contact with the fifth layer, is a sealant layer. The sealant layer may comprise any of several polymers commonly used in sealant layers, including LDPE, LLDPE, ethylene vinyl acetate (xe2x80x9cEVAxe2x80x9d), ethylene methyl acrylate (xe2x80x9cEMAxe2x80x9d), ethyl methyl acrylic acid (xe2x80x9cEMAAxe2x80x9d), an ionomer, or a blend of any of these polymers.
The first embodiment may alternatively also include an eighth layer disposed in contact with the sixth layer of nylon. The eighth layer comprises an anhydride modified polyolefin, of the type that is commonly used as a polymeric adhesive.
In a second embodiment, the film comprises first and second core layers of nylon. As in the first embodiment, the nylon layers preferably comprise from 5 to 35 percent by weight of an amorphous nylon copolymer that is blended with one or more of various other nylons. Each of the first and second nylon layers may also comprise a nucleating agent.
Disposed between the first and second layers of nylon is a third layer of an adhesive or tie resin. The adhesive resin may be a anhydride modified polyolefin, such as an EVA-based or LLDPE-based adhesive, or any of the various other polymeric adhesives commonly used in the art of making multilayer films. Fourth and fifth layers of the adhesive are disposed in contact with the first and second layers of nylon, respectively.
A sixth outer layer of a nylon is disposed in contact with the fourth layer of an adhesive. Similar to the first and second layers of nylon, the sixth layer preferably comprises from 5 to 35 percent by weight of an amorphous nylon copolymer that is blended with one or more of various other nylons, and may include a nucleating agent and an antiblocking agent.
The seventh layer of the film, the sealant layer, preferably comprises a blend of LLDPE and LDPE. The sealant layer may also comprise EVA, the linear polyethylene ULDPE, EMA, EAA, EMAA, an ionomer, or blends of any of these polymers.
In a third embodiment, the film comprises a first core layer of an EVOH copolymer with up to 44 mole percent ethylene. The first layer is sandwiched between second and third layers of nylon. As in the previous embodiments, the second and third layers preferably comprise from 5 to 35 percent by weight of an amorphous nylon copolymer, that is blended with one or more of various other nylons. Each of the second and third layers may also include a nucleating agent. A fourth outer layer of nylon, which like the second and third layers preferably comprises from 5 to 35 percent by weight of an amorphous nylon copolymer that is blended one or more of various other nylons, is disposed in contact with the second layer of nylon. The fourth layer of nylon may also include a nucleating agent.
A fifth layer of an adhesive or tie resin is disposed in contact with the third layer of nylon. Suitable adhesive resins include anhydride modified polyolefins, such as EVA-based or LLDPE-based adhesives.
Disposed in contact with the fifth layer of an adhesive is a sixth layer of an ionomer. Disposed in contact with the sixth layer is a seventh layer, a sealant layer, which may comprise LLDPE. The seventh layer may alternatively comprise EVA, LDPE, ULDPE, EMA, EMAA, EAA, an ionomer, or blends of any of these polymers.
In a fourth embodiment of the seven layer film, the film comprises a first core barrier layer of an EVOH copolymer having up to 44 mole percent ethylene. The core layer is sandwiched between second and third layers of nylon. As in the previous embodiments, the second and third layers preferably comprise from 5 to 35 weight percent of an amorphous nylon copolymer that is blended with one or more other nylons, and may further include a nucleating-agent.
Fourth and fifth layers of an adhesive or tie resin are disposed in contact with the second and third layers of nylon, respectively. Suitable adhesive resins include anhydride modified polyolefins, such as EVA-based or LLDPE-based adhesives, or any of the various other polymeric adhesives that are commonly used in making polymeric films.
Disposed in contact with the fourth layer of an adhesive is a sixth outer layer. The sixth outer layer may comprise a polypropylene, polyester, any of the polyethylenes manufactured by way of a single site catalyst, medium density polyethylene (xe2x80x9cMDPExe2x80x9d), linear medium density polyethylene (xe2x80x9cLMDPExe2x80x9d), LDPE, LLDPE, EVA, styrene including rubber-modified styrene, or a blend comprising any of these polymers.
A seventh layer, of a sealant, is disposed in contact with the fifth layer. The sealant layer may comprise EVA, LDPE, LLDPE, ULDPE, EMA, EMAA, an ionomer, or blends of any of these polymers.
In a fifth embodiment, the films of the invention comprise a first core barrier layer of an EVOH copolymer having up to 44 mole percent ethylene. The core layer is sandwiched between second and third layers of nylon. As in the previous embodiments, the second and third layers preferably comprise 5 to 35 weight percent of an amorphous nylon copolymer that is blended with one or more various other nylons. Each of the second and third layers may also include a nucleating agent.
A fourth layer of an adhesive or tie resin is disposed in contact with the second layer of nylon. The adhesive layer may comprise anhydride modified polyolefins, such as EVA-based or LLDPE-based adhesives, or any of various other polymeric adhesives that are commonly used in the making of polymeric films.
A fifth outer layer is disposed in contact with the third layer of a nylon. The fifth layer comprises an anhydride modified polyolefin, of the type that is commonly used as a polymeric adhesive.
Disposed in contact with the fourth layer of an adhesive is a sixth layer of a bulk olefin resin. The sixth layer may be a bulk or general olefin resin of various polymeric substances, particularly low-priced resins. For example, the sixth layer may be a regrind of various scrap polymeric materials. Alternatively, the sixth layer could comprise a variety of low cost resins, including EVA, or one of the various types of polyethylene, including LDPE and LLDPE, or a blend of any of the aforementioned polymers.
Disposed in contact with-the sixth layer of a bulk olefin is the seventh layer of a sealant. The sealant layer may comprise EVA, LDPE, LLDPE, ULDPE, EMA, EMAA, an ionomer or blends of any these polymers.
The films of the invention may be of any thickness. A preferred range of thickness is from 2 to 10 mils, and a most preferred range is from 2.5 to 7.5 mils.